This invention relates to the imaging arts, and more particularly, to range imaging employing parallax.
With the increasing utilization of robots in industry and the more sophisticated tasks being performed by robots, a strong need has been felt for improved visual sensors. Initial attempts to provide robots with vision imitated the human visual process, employing intensity imaging. While steady progress has been made, it has been slow and requires a basic theoretical breakthrough before this type of vision reaches its full potential in robotics. The current state-of-the art is just entering real-time gray scale processing for robots and has been only marginally useful in military reconnaissance and industry.
To overcome the problems inherent with intensity imaging or the like, range imaging has been employed. Although a number of different range imaging techniques has been proposed, employing parallax appears to be the most promising.
A number of parallax range imaging systems are known in the art. For example, in U.S. Pat. No. 3,866,052 to DiMatteo et al, the range between a predetermined point and a field including a scene to be imaged is divided into a series of segments. Four different light patterns are projected onto the field sequentially and intensity data reflected from the scene is obtained. This intensity data is then combined to determine in which of the segments each of the elements of the scene lies. A problem with this system is that resolution is limited to the size of the segments.
U.S. Pat. No. 4,145,991 to DiMatteo et al teaches a range imaging system with improved resolution. In this patent, as in DiMatteo '052, the scene is subdivided into a number of segments. Resolution is improved by overlapping the subdivided segments so that a portion of one segment overlaps a portion of a neighboring segment. Resolution is maximized when the number of segments is increased.
In this patent, the scene is illuminated with a number of different modulation patterns and reflected intensity information is employed to determine the range of points in the scene. According to this patent, the resolution that may be obtained is related to the number of different modulation patterns projected onto the scene. Thus, this patent is similar to the DiMatteo et al '952 patent in that range is divided into a number of segments and resolution is limited to determining in which segment a point of the scene lies. This patent does provide the additional teaching that resolution can be improved by decreasing the size of the segments, which in turn is accomplished by increasing the number of modulation patterns projected onto the scene.
U.S. Pat. No. 4,259,017 to Ross et al and the parent application both teach further improvements on parallax based range imaging techniques. Specifically, these teach range imaging techniques in which the range can be determined continuously. Instead of employing illumination patterns which are binary (i.e., either transmissive or opaque), sinusoidally varying illumination patterns are employed. Four frames of reflected intensity data are collected to produce a range image.
The techniques taught by Ross et al and the parent application are greatly improved over the prior art with respect to resolution. However, certain problems do exist. Specifically, four time spaced frames of intensity data must be collected. The more frames of intensity data that must be collected, obviously the more time must be consumed to produce one frame of range data. The necessity of collecting four frames of data may cause problems if a fast moving scene is being monitored and real-time processing is required.
Also, in Ross et al and certain embodiments of the parent application, the four frames of data are stored in memory. Obviously, the more frames of data that must be stored, the larger the memory must be, increasing the cost of the apparatus.
Also, the parent application suggests that a square wave Ronchi pattern may be employed to approximate a sinusoidal pattern. When projecting a moving Ronchi pattern, the third harmonic is the most serious degrading factor in measurement accuracy. To improve this situation, the parent application suggests defocusing the Ronchi pattern. However, the problem with the third harmonic still exists.